If plastic, I had good experience with JBWeld for plastic. I feathered it in a crack or hole and sanded. I painted with a sealer and then color matched paint for a headliner crack. Came out pretty nice.
You can easily add one to the boat and it was an option when new. They drill the hull and install a tunnel for the thruster. I'd say you are looking at $12-15K.
Most early boats had the railing that was in pieces and was held together at the intersections points with female adapters and set screws. I prefer it because replacements can be done easily, low piece part cost and low shipment cost due to knock down design. The one piece ones sure look...
Can't use starboard or any plastic board, it needs to be structural to hold the weight. I'd use 3/4" plywood, router everything and support on the backside with wood support strips (glued and screwed). The big thing will be weight and storage when not desired.
Could try A&J Canvas in Vanceboro NC. I believe they make them for GW. Have the boat model, year and dimenstions/pics. If the existing can be used as template, ship it to them.
once you get above 30' the GWs price seems to escalate dramatically. You have to look at other brands. There are a lot of options out there for various use cases. I love my GW buy if buying new, I'd probably be going in another direction.
Good luck, personally I couldn't waste the time to film/edit all the things I've fixed, re-engineered or replaced in my lifetime. I appreciate those that do, but only watch what I need to.
The typical spots for water getting in are leaks at the stanchions for the hardtop, windows, cup holders (drain into bilge), hatch inspection ports (replace seals, replace with screw in version vs pop-in, and re-caulk to bulkhead.